1. Field of the Invention
The invention relates to multilayer polyolefin film structures and methods of making the same and, more particularly, the invention relates to a biaxially-oriented polypropylene film incorporating at least one polyester layer.
2. Description of Related Art
Biaxially-oriented polypropylene (BOPP) films are widely used in packaging because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Users of packaging films, particularly users of biaxially oriented polypropylene films, are continually seeking structures with improved printability, metallizing properties, and gas barrier. Because of their olefinic nature, typical BOPP constructions have low surface energy and require treatment (corona, flame, etc.) in order to be printed or metallized. Polyester is known to have high surface energy and possesses excellent printing and metallizing attributes. Additionally, polyester, both in clear and metallized structures offers improved gas barrier performance to BOPP films. This is especially true in the case of metallized polyester films which are in order of magnitude or more lower in oxygen transmission rate.
Although there has been a long felt need for BOPP films incorporating polyester layers, problems in obtaining acceptable optical and processability characteristics, adequate interply adhesion, and other properties have been encountered in previous attempts to produce polyester-containing BOPP structures. For example, U.S. Pat. No. 5,324,467 a process for the preparation of an oriented multilayer laminate film having at least three layers, including polypropylene, a tie layer, and copolyester is disclosed. The films are formed by combining the layers in the molten state, either in coextrusion, or in separate extrusions brought together outside the die, then subsequently cooling the film, orienting it uniaxially or biaxially, and heat setting to lock in the properties. A major problem in producing a structure according to this method on commercial scale equipment is the strong tendency of polyester to adhere to the heated metal rolls of the machine direction orientation section. This makes it difficult to achieve good optical properties free of visual defects and may also decrease other properties such as the seal initiation temperature.
U.S. Pat. No. 4,874,656 describes a multilayer laminate having a high mechanical resistance and an impermeability to gases and vapors. In the disclosed structures, a polyester layer is joined to a polypropylene layer after the polypropylene is biaxially-oriented, the polyester layer is quite thick (i.e., 12 to 24 microns), and the structure includes a layer of metallic foil and a layer of polyethylene. While joining a polyester layer to a BOPP layer after biaxial orientation is possible, this method is impractical for incorporating thin layers of polyester. U.S. Pat. No. 4,924,525 also describes a structure wherein a polyester laminate is adhered to a BOPP film after the polypropylene is biaxially oriented, precluding the use of thin polyester layers in the final structure.